It is well known that pitch ununiformity constitutes a significant factor adversely affecting the quality of a scanning line image obtained by modulating and deflecting a light beam such as a laser beam and forming scanning lines of the light beam on a light-sensitive material to record the image thereon. By "pitch ununiformity" is meant a stripe pattern visually appearing in an image formed by scanning lines of a light beam due to ununiform intervals among the scanning lines in a portion of the image wherein the intervals among the scanning lines are not uniform. As the causes for pitch ununiformity, two problems with regard to the apparatus have heretofore been known. One is the feed ununiformity in sub-scanning as described, for example, in the paper entitled "Problems in Sub-scanning in Cylinder Type Facsimilie" in National Technical Report, Vol. 22, No. 5, Oct. 1976, pages 55 to 558. The other is the face inclination of a light deflector as described, for example, in the paper entitled "System for Correcting Scanning Line Pitch Ununiformity in Multi-face Rotating Mirror Facsimile Scanning" in Gazo Denshi Gakkaishi (Image Electronics Society Magazine), Vol. 5, No. 3, 1976, pages 107 to 113.
However, the inventors found that, when main scanning is conducted at a relatively low speed of 1 kHz or less since the amount of the image information is enormous and sub-scanning is conducted at a high scanning density of 2.5 lines/mm or more and when an attempt is made to conduct read-out or reproducing of a half-tone radiation image at a high accuracy by point-by-point scanning, pitch ununiformity still occurs even if the aforesaid causes for pitch ununiformity, i.e. the sub-scanning feed ununiformity and the face inclination of the light deflector are eliminated completely. (When pitch ununiformity occurs at the time of image read-out, pitch ununiformity also occurs in the image reproduced from the read-out image.) A radiation image used for medical diagnosis must have a quality allowing accurate diagnosis. Namely, the radiation image must have a quality suitable for viewing, particularly for diagnostic purposes. However, when pitch ununiformity is generated in the radiation image, it becomes impossible to accurately conduct diagnosis and, in the worst case, there is a risk of the radiation image being diagnosed erroneously.
The inventors conducted various experiments to eliminate pitch ununiformity generated by a third cause other than the two causes described abouve when reading out and reproducing a half-tone radiation image at a high accuracy, and found that the third cause is an air stream generated in the optical path of the light beam between a laser beam source and a medium for read-out or reproducing. Namely, when an air stream is generated in the optical path of the light beam, the air density becomes ununiform and, therefore, the refractive index is changed slightly. As a result, the light beam is subjected to refraction, and pitch ununiformity is generated on the surface scanned with the light beam.
Causes for the generation of the air stream involve heat convection due to heat generated by a light beam generating means, and air vibrations due to a cooling fan for cooling the light beam generating means. Since the swinging frequency of the air stream is within the range of several tens of hertz or less, the air stream adversely affects the radiation image read-out and reproducing wherein the main scanning is conducted at a relatively low speed of 1 kHz or less as described above.
The object of the present invention is to provide a radiation image read-out and reproducing apparatus which prevents an air stream from being generated in the optical path of a light beam and conducts read-out or reproducing of a radiation image suitable for viewing, particualrly for diagnostic purposes without generating pitch ununiformity.